This invention relates to screw rotors of screw compressors, and more particularly, to rotor tooth forms for a pair of screw rotors suitable for use with a dry type screw compressor, blower, expander, etc., in which the rotors rotate while in meshing engagement with each other by using synchronizing means without being brought into contact with each other.
Generally, in oil less type screw compressors for use in applications where mingling of oil in the gas discharged from a screw compressor is not desirable, transmission of rotation between a pair of screw rotors is effected through synchronizing means mounted at shaft portions outside the working chambers of the rotors, and at this time the rotors rotate while meshing with each other without coming into contact with each other. The screw rotors of this type of screw compressors have a greater chance of the gas leaking from between the rotors and the casing and between the rotors and from the blowholes than the screw rotors of screw compressors of the oil-cooled type in which the oil is injected into the working chambers in which the rotors mesh with each other to effect lubrication and cooling of the rotors and provide a seal to the rotors. As a result, the size of the clearance and the blowholes exert great influences on the efficiency of the compressors. In view of this fact, there has been a demand for a high degree of accuracy and precision with which the rotors are shaped and for a rotor tooth form of small blowholes.
In the screw rotors of this type of screw compressors, the teeth of the rotors have their temperatures raised to a high level during operation, and, consequently, are greatly deformed during operation as compared with the rotor teeth in normal temperature during an inoperative period. Thus, in designing the shape of the two rotors of a screw compressor, it is necessary not only to take into consideration the dimensions of the rotors to provide clearances between the rotors and between the rotors and the casing in such a manner that the rotors are not brought into contact with each other during operation and yet the clearance is minimized, but also to provide means for avoiding the occurrence of seizure between the rotors and the casing.
In designing screw rotors, it has been the usual practice to decide a clearance between the two rotors and a clearance between the rotors and the casing based on a casual idea, and, consequently, the clearances formed have had no theoretical basis. This has given rise to a number of problems with regard to the operation efficiency of the screw compressors that have remained unsolved.
More specifically, as a process for imparting a clearance between the two rotors, proposals have been made to use a male rotor as a reference for providing a basic tooth form of the rotors and a clearance of a predetermined size is provided in the direction normal to the female rotor tooth form by taking into consideration deformation and other factors that might possibly be caused to occur by thermal expansion during operation. The screw rotors produced by this process have already been put to practical use.
However, in view of the fact that deformation of the tooth form due to thermal expansion may vary depending on the shape of the tooth form, the value of the clearance decided by the process described hereinabove would not be considered an optimum value that is obtained by careful analysis of the condition of the rotors expanded by heat and of the condition of the clearance during operation.
In, for example, U.S. Pat. No. 3,414,189, another process for imparting a clearance to the two rotors of a screw compressor is proposed wherein a small clearance is provided in a region in which the relative sliding movement between the two rotor teeth meshing with each other is small and a sufficiently large clearance is provided to other regions of the rotor teeth.
However, the last-mentioned process could not be considered to take the thermal deformation of the two rotors into consideration quantitatively in providing a clearance to between the rotor.